1. Field of the Invention
This invention relates to an amplitude-modulated transmitter in which a carrier value where carrier value is understood to refer generally to the level of the transmitted carrier signal and more particularly to the ratio of the actual carrier voltage or current divided by a maximum carrier voltage or current, respectively, is controlled by the modulation level which is understood to refer to the level of the modulating signal strength, i.e., the actual modulation signal amplitude divided by the maximum modulation signal amplitude.
2. Description of the Prior Art
For the purpose of reducing the power costs of an amplitude-modulated radio transmitter, and taking into consideration the receiver characteristics, a method for modulation with variable carrier value has been proposed quite early (Radio-Frequency Engineering and Electro-Acoustics, Jahrbuch der drahtlosen Telegraphie und Telephonie, volume 47, number 5, May 1936, page 141 and following). This method, known as HAPUG (controlled-carrier) modulation, provides linear control of the carrier value as a function of the level of the modulation signal which controls the carrier value forward from a residual carrier value at a modulation level of zero up to its maximum value at maximum modulation value. The residual carrier value limits the effect of a curved detector characteristic in the receiver on the quality of the demodulated radio signal.
On the basis of the HAPUG method it was furthermore been proposed (German Patent No. 3,037,902) to increase the energy saving by keeping the characteristic of the carrier control below the HAPUG characteristic, at least in the region of the most frequently occurring modulation levels whilst keeping the residual carrier value the same. In the extreme case, the carrier value then remains constant for lower modulation levels and identical to the residual carrier value and is forward-controlled only above a certain level value along the straight line resulting for a modulation percentage of 100%. In order to reduce the risk of over modulation, the steeper part of the characteristic can also be displaced in parallel towards smaller level values.
The two known methods for carrier control have one thing in common: a direct connection exists between the magnitude of the residual carrier value and the energy saving achieved. The smaller the residual carrier value selected, the greater the energy saving. On the other hand, the reduction of the residual carrier value is accompanied by a reduction in the signal-to-noise ratio at the reception site which leads to a noticeable deterioration in the quality of reception.
A weakening of the carrier to 60% of nominal amplitude (0.6 residual carrier value) as has been proposed in German Patent No. 3,037,902 is much too great for present and future conditions in the broadcast bands with their high density of transmitters. This reduces the signal-to-noise ratio by about 4.4 dB which represents an evident reduction in quality in the fringe areas of a service area especially during intervals and quiet passages of music. However, if the carrier is not reduced by as much, the desired energy saving is not achieved.